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Date: Fri, 10 Feb 2006 02:42:22 -0800
From: Andrew Morton <akpm@osdl.org>
To: Andi Kleen <ak@muc.de>
Cc: ashok.raj@intel.com, ntl@pobox.com, dada1@cosmosbay.com, riel@redhat.com,
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Subject: Re: [PATCH] percpu data: only iterate over possible CPUs
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Andi Kleen <ak@muc.de> wrote:
>
> On Thursday 09 February 2006 19:04, Andrew Morton wrote:
> > Ashok Raj <ashok.raj@intel.com> wrote:
> > >
> > > The problem was with ACPI just simply looking at the namespace doesnt
> > >  exactly give us an idea of how many processors are possible in this platform.
> > 
> > We need to fix this asap - the performance penalty for HOTPLUG_CPU=y,
> > NR_CPUS=lots will be appreciable.
> 
> What is this performance penalty exactly? 

All those for_each_cpu() loops will hit NR_CPUS cachelines instead of
hweight(cpu_possible_map) cachelines.

> It wastes quite some memory (each possible CPU needs 32K of memory which
> adds quickly up), but it shouldn't impact other CPU use. 
> 
> > 
> > Do any x86 platforms actually support CPU hotplug?
> 
> Xen does.

yup.

> And it's needed for suspend/resume on normal x86 now.

True.